Experimental results using a “low-voltage booster” in a conventional SEM

A “low-voltage booster” (LVB), which was proposed by E. Plies [US Patent 5,146,090, September 8, 1992; see also S. Beck et al., Nucl. Instr. and Meth. A 363 (1995) 31], is adapted to a conventional medium-voltage scanning electron microscope (SEM) to improve its resolution for low-voltage operation. The LVB consists of a tube electrode which is charged with a high positive potential and is arranged between the last intermediate image and the pole-piece gap of the final magnetic objective lens. Since the tube electrode extends into the region of the pole-piece gap, an electrostatic retarding field is overlaid on the focussing magnetic field and the compound lens field has low chromatic and spherical aberrations. Using the LVB, we succeeded in improving the resolution of the SEM by a factor of 3 for 700 eV and 2 for 1.5 keV beam energy.